Page 53 - Industrial Wastewater Treatment, Recycling and Reuse

36 Industrial Wastewater Treatment, Recycling, and Reuse

an effective water treatment method (Ranade et al., 2006, 2013). Further,
cavitation can offer considerable economic benefits compared to other con-
ventional physico-chemical methods of treatment.
Cavitationistheformation,growth,andcollapseofcavities,releasingalarge
amountofenergyandgeneratingoxidizing agentsinwastewater.Cavitiesgen-
eratedundergoaseriesofradicalreactionswithcomplexorganicmatterpresent
in wastewater leading to the destruction of contaminants and decolorization of
wastewater. Physico-chemical changes in the fluid take place due to transient
temperature and pressure conditions with strong oxidizing conditions as active
chemical radicals and hydrogen peroxide are formed. The process is somewhat
analogous to advanced oxidation processes (AOPs), with local high magnitude
pressure pulse, 100–5000 atm., and extremely high temperatures of 1000–

15,000 K. However, the overall liquid medium can be maintained close to
ambient conditions. The cavitation technology can be combined with an array
of other technologies such as Fenton, ozone, wet air oxidation, and others for
providingaveryeffectiveplatformforsolvingavarietyofwastewatertreatment
problems. Thus, many hybrid technologies are possible, such as cavitation
+oxidation; cavitation+coagulation; cavitation+adsorption/ion-exchange;
cavitation+membrane, and cavitation+biological treatment.
The cavitation technology not only offers promising methodology on
its own for industrial wastewater treatment, but it can also be integrated
effectively with other conventional methods for achieving a complete,
techno-economically feasible solution for water recycling and reuse.

1.4.6 Advanced Oxidation Processes

AOPs have been in commercial practice, especially for refractory pollutants
that are difficult to remove using conventional physico-chemical methods.
These are mainly useful for highly toxic and non-biodegradable wastes.
Although there is significant variation in the form of catalyst and reactor
configurations used for OPs, Fenton oxidation and photo-Fenton have been
more successfully applied in industrial wastewater treatment. AOPs operate
through the generation of hydroxyl radicals and other oxidant species to
degrade organic compounds in wastewater. There are different methods
for the generation of hydroxyl radicals, which include:
• H 2 O 2 (Fenton)/O 3
• Hydrodynamic and acoustic cavitation
• Homogeneous ultraviolet irradiation
• Heterogeneous photocatalysis using semiconductors
• Radiolysis/electric and electrochemical methods.